In the field of metal processing, it is well known that certain metals such as titanium are highly reactive in the molten state, and are conducive to refinement by remelting in crucibles. To prevent undesirable reactions with the crucible material, the well known "skull-melting" method was developed, in which the liquid working metal is contained in a solid shell or "skull" of the same metal, which in turn is surrounded by a water-cooled hearth. Heating is provided by a plasma torch disposed above the working metal surface, the plasma torch being moved about the surface area to provide uniform transfer of heat to the working metal.
For many processes, such as the continuous or semicontinuous plasma remelting of titanium, it is important to know the volume of molten working metal in the hearth. A certain minimum residence time in the liquid state must be maintained in order to decompose low-density inclusions, which would limit the strength of the metal. To determine the volume of working metal in the molten state, it is necessary to know both the level of the molten pool and the thickness of the solid skull underneath. Access to the top surface of the pool of molten working metal is limited by the moving plasma torch above the surface. Therefore, ultrasonic devices have been placed on the underside of the hearth to obtain measurements indicative of liquid depth and skull thickness.
At the same time, however, in order to minimize heat loss to the hearth, a rough, low density, low heat conductivity interface is intentionally created between the hearth and the skull. Ultrasonic devices placed under the hearth are thus rendered ineffective because the interface prevents sonic coupling to the skull. A means for coupling a transducer directly to the skull is thus required for successful transmission of the ultrasonic signal for accurate, precise liquid depth measurement.